what to do?? updated

so my bf and I have been together a year and are expecting a baby just recently he doesn't want sex it went from 3-4 times a week to once a month should I be worried or is this normal after a year? ps I know it's not that I'm pregnant he was fine having sex in the second trimester I just started my third. also if I try anything he makes excuses like I'm tired etc. any input helps thank you. **update I asked and he said what three or four times a week isn't good enough so I corrected him it's once a month and he just looked down and acted busy and wouldn't talk to me. I also went through his phone nothing but it's weirdly clean like he never uses it he also doesn't have me as anything but my name with his exs they had cute nicknames I have nothing. he is 37 yrs old.